A DC-to-DC converter is a circuit or device which converts DC power from one voltage to another voltage. A DC-to-DC converter accepts a DC input voltage (VIN) from a power source and produces a DC output voltage (VOUT) for use by a load. Typically the DC output voltage (VOUT) produced is at a different voltage level than the DC input voltage (VIN). For instance, a DC-to-DC converter may convert a relatively low input battery voltage (VIN) to a higher DC output voltage (VOUT), or vice versa, and in some cases converts the input battery voltage (VIN) to a negative DC output voltage (VOUT). A normal DC-to-DC converter typically uses feedback about the DC output voltage (VOUT) to regulate the DC output voltage (VOUT).
In some applications, the load will consume as much input current (IIN) from the power source as it requires to supply an output voltage (VOUT), and the power source will provide as much power as needed to supply this input current (IIN). One such example is illustrated in FIG. 1 which is a circuit schematic that illustrates a circuit 105 for supplying an output voltage (VOUT) to a load 190. The circuit 105 includes a fuel cell 110 coupled to a DC-to-DC converter 120. The DC-to-DC converter 120 receives a DC input voltage (VIN) from the fuel cell 110 and converts it to a DC output voltage (VOUT) for use by the load 190.
The fuel cell 110 is represented by a voltage source 112 having a relatively high series resistance 114 and capacitance 116. The fuel cell 110 provides an input voltage (VIN) across capacitance 116. The value of the input voltage (VIN) is equal to the difference between a voltage 112 of the fuel cell 110 and the voltage drop across series resistance 114. The series resistance 114 is “high” relative to the voltage source 112 since product of the input current (IIN) and series resistance 114 can be up to fifty percent (50%) of the voltage source 112. The load 190 is represented as load resistance 194 in series with a load voltage 196. An output voltage (VOUT) provided across the load resistance 194 and load voltage 196 provides power to the load 190. The resistance value of load resistor 194 may vary with the state of the load.
The DC-to-DC converter 120 receives a variable input voltage (VIN) and “ideally” regulates the value of output voltage (VOUT) so that the output voltage (VOUT) remains as close as possible to a desired value specified by a reference voltage (VREF) 132.
When the load 190 is powered, the DC-to-DC converter 120 will attempt to supply as much power as needed to supply the load 190 with the desired output voltage (VOUT). As the input current (IIN) drawn by the DC-to-DC converter 120 increases, the voltage drop across the input series resistance 114 also increases, and therefore the input voltage (VIN) to the DC-to-DC converter 120 decreases. In some situations, the load 190 attempts to consume more input power than is available from the fuel cell 110. If the input current drawn by the load 190 becomes very high, then the input voltage (VIN) will drop so much that output power available to the load 190 will actually decrease despite the fact that the load 190 is actually drawing more input current (IIN). As a result, the output voltage (VOUT) can fall below a desired value (e.g., less than reference voltage (VREF) 132). This problem is exacerbated by the fact that the fuel cell 110 has a relatively high series input resistance 114 in comparison to the power delivered.
For instance, consider an example where the output voltage (VOUT) is to be regulated at 5.0 volts, the source voltage 112 of the fuel cell 110 is 0.9 volts, the reference voltage (VREF) is 5 volts, and the fuel cell's internal resistance 114 is 0.125 ohm. When the load resistance 194 is at 25 ohms, the load current is 200 milliamperes, and the load 190 consumes a total output power (POUT) of 1 watt. Assuming 90% efficiency, the input power (PIN) would be 1/0.9=1.11 watts, which means that the fuel cell 110 must provide an input current of 1.58 amperes at an input voltage is 0.70 volts. The fuel cell 110 can supply this amount of input power. However, the load resistance 194 may change. For example, if the load resistance 194 decreases to 12.5 ohms, then the output power (POUT) needed to maintain the same 5 volt output voltage (VOUT) at the load increases to 2 watts, which would require 2.22 watts of input power (PIN) from the fuel cell 110. Even though the fuel cell 110 cannot supply this much input power, the load 190 will nevertheless attempt to consume more input current (IIN), which in turn causes the input voltage (VIN) to decrease further. As a result, the system becomes unstable and is unable to deliver power efficiently. This point occurs when the voltage supplied by the fuel cell 110 to the DC-to-DC converter 120 falls to one-half of the unloaded value. Beyond this point, the power delivered to the DC-to-DC converter 120 actually decreases as the input current (IIN) increases, and also consumes excessive current from the fuel cell 112.
Thus, although the DC-to-DC converter 120 can regulate the amount of power consumed from the fuel cell 110 to provide a desired value of the output voltage (VOUT), there is no mechanism for simultaneously preventing the input voltage (VIN) from falling below a value capable of providing the desired output power (POUT).
Accordingly, it is desirable to provide an improved DC-to-DC converter designed to prevent the input voltage (VIN) of a power source from decreasing so much that the input voltage (VIN) falls below a minimum desired value. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.